The present invention is a continuous, economical, and efficient method for separating carbon from fly ash, such that the remaining fly ash composed generally of mineral matter is suitable for use in applications including the manufacture of cement, and a concentrated carbon product is recovered.
Fly ash is a by-product from the combustion of pulverized coal and is comprised of organic and inorganic fine ash particles. The combustion process fundamentally transforms the organic and inorganic material of the coal and produces fly ash having unique physical properties and a chemical composition very distinct from the coal starting material. For example, coal combustion does not simply concentrate the inorganic mineral matter by burning off carbon, but rather changes the inorganic mineral matter into a new material comprising fused beads of inorganic ash, while liberating soluble ions not present in the original inorganic coal phase. The large quantities of soluble ions present in fly ash, e.g., sodium and calcium, dramatically influence the chemistry of fly ash in aqueous solutions. The high temperatures of coal combustion also burn away volatile materials, in addition to a substantial amount of carbon, present within the coal, leaving a more stable organic char comprised of cross-linked carbon material having a highly graphitic structure not present in coal. The char (organic) phase in fly ash is significantly different from the organic material in coal, as the char has a larger surface area, higher aromaticness, and a lower hydrogen content. Fly ash, therefore, has a physical structure, chemical composition, aqueous solubility, and surface reactivity that is distinct from coal.
Fly ash is currently generated in very large quantities from coal combustion in power plants (millions of tons a year in the U.S.). In the past, the generated fly ash had a low carbon content of less than 6 wt % and was therefore suitable as a partial cement replacement and/or mineral additive in cement. Although the majority of/fly ash is deposited in landfills, the use of fly ash in cement production alleviates expensive waste disposal problems and beneficially increases the quality of the cement product, adding strength and increasing sulfate resistance, while contributing to a more economical cement production process. Recent clean air regulations, however, require electric utility power plants to reduce nitrogen oxide (NOx) emissions, and low NOx burners have the unfortunate side effect of generating fly ash having an unburned carbon content of between 6 wt % to 25 wt %, unacceptably high for cement production. To convert high carbon fly ash into a useful by-product, the carbon content must be reduced. In addition to generating a useful low carbon fly ash, it would further be advantageous for methods for reducing the carbon content in fly ash to produce a residual carbon product that may be utilized for recombustion and energy production, or as a catalytic material.
Unsurprisingly, several methods for reducing the carbon content in fly ash have recently been disclosed. Many of these methods are dry processes involving thermal treatments that combust the unburned carbon to produce low carbon fly ash. For example, U.S. Pat. No. 5,749,308 issued to Bachik describes igniting the carbon by a heated oxidizing gas stream in an ignition chamber and transferring the fly ash and ignited carbon to a combustion chamber to accomplish the desired level of carbon burnout. U.S. Pat. No. 5,555,821 issued to Martinez teaches moving the fly ash through a stainless steel heating chamber by a screw auger, which mixes the fly ash with oxygen to burn the unwanted carbon. U.S. Pat. No. 5,390,611 issued to John describes a dry thermal process involving tumble mixing the fly ash through electrically heated pre-heat and combustion chambers. Finally, U.S. Pat. No. 4,663,507 issued to Trerice teaches employing microwave energy to induce combustion of the carbon in the fly ash.
Combusting unburned carbon in fly ash is energy intensive and therefore inherently expensive. Combustion methods are also often very sensitive to the variability in the fly ashes derived from different coal sources and involve large air handling systems that require expensive gas/particulate separators. In addition, combusting the carbon may result in agglomeration of the fly ash, which is undesirable when the fly ash is used for cement production, and any valuable carbon by-product is lost.
U.S. Pat. No. 5,513,755 issued to Heavilon, et al. discloses another dry process, wherein heated fly ash is moved along a belt-type vibrating conveyor through an electrostatic charging zone which causes the carbon particles to become charged and attracted to an electrode for separation from the fly ash. Electrostatic dry particle separation methods have been generally impractical for carbon reduction in fly ash due to the special characteristics of fly ash, including the large quantity of very fine particles which comprise a substantial fraction of the carbon content. U.S. Pat. Nos. 5,339,194 and 5,160,539 issued to Cochran, et al. also discloses a dry process involving oxidation of the carbon in fly ash in dry, bubbling fluid beds at high temperatures. Other dry processes involve classification by particle size, including methods described in U.S. Pat. No. 5,299,692 issued to Nelson, wherein fly ash is subjected to a vibrating inclined surface to disaggregate and stratify a high carbon fraction from the fly ash, and in U.S. Pat. No. 3,769,054 issued to Pennachetti, wherein fly ash is subjected to air classification and screening, among other steps.
Proposed wet processes for separating carbon from fly ash are known to involve froth flotation methods that conventionally separate mineral matter by use of specific reagents and chemical conditions. Froth flotation plants include conditioning tanks and flotation vessels having a series of flotation cells. The use of a flotation liquid, a collector, a frother, and possibly dispersants, activators, and other regulation agents, are necessary. Generally, a chemical reagent is added to the mineral slurry in the conditioning tank to make certain mineral surfaces hydrophobic by absorption, while leaving other mineral surfaces hydrophilic. The slurry is aerated in the flotation vessel, whereby the air bubbles become laden with the hydrophobic particles and rise to the surface. The separated mineral particles are transferred to the next flotation cell until sufficient separation is achieved. Advantageously, the carbon is not consumed as in combustion by the wet process of froth flotation, but recoverable as a valuable by-product.
U.S. Pat. No. 5,456,363 issued to Groppo, et al. describes a method of removing carbon from fly ash by an improved froth flotation method. The method includes producing an aqueous slurry, adding a flotation reagent comprised of a mixture of fuel oil and petroleum sulfonate to make the carbon hydrophobic, aerating the slurry, and recovering the carbon from the upper portion of the flotation apparatus, while withdrawing the fly ash tailings from the lower portion. Similarly, U.S. Pat. Nos. 5,227,047 and 5,047,145 issued to Hwang teach a wet process for fly ash beneficiation, wherein the unburned carbon is separated from a fly ash slurry by adding to the slurry a collector (oil having a carbon chain greater than octane), a dispersant, and a frothing agent, and inducing air into the system for frothing the slurry wherein the hydrophobic unburned carbon froths to the surface and is removed by skimming off the frothing layer.
Froth flotation is a complex process influenced by pH, slurry density, particle size, bubble size, air flow, reagent mixtures, and efficient separation and recovery methods. Inefficiencies associated with froth flotation as applied to carbon separation in fly ash include the use of excessive amounts of reagents required to render carbon particles hydrophobic, lengthy flotation time, and inadequate recovery of a significant amount of the separated material (carbon). These inefficiencies are most probably caused by the unique characteristics of fly ash, including the fineness of the fused beads of material and large surface areas of the carbon particles. Flotation also requires a significant investment in capital equipment and high operating expenses.
In view of the above discussion, there continues to exist a need in the art for an efficient and economic method for reducing the carbon content in fly ash.
The present method is a continuous process for separating carbon from the mineral compositions in fly ash by using an air agglomeration process.
Therefore, in view of the above, a basic object of the present invention is to provide a continuous, cost-effective, and efficient method for reducing the carbon content in fly ash, whereby the fly ash is useful in cement production.
A further object of this invention is to provide a method for recovering a valuable concentrated carbon product from fly ash.
Yet another object of this invention is to provide a method for recovering a concentrated carbon product from fly ash having a high yield, or ratio of recovered carbon to total carbon in the untreated fly ash of greater than 60 wt %.
Yet another object of this invention is to provide a method for recovering a concentrated carbon product from fly ash having a high purity, or ratio of recovered carbon to total recovered material of greater than 60 wt %.
A further object of this invention is to provide a continuous method for recovering a concentrated carbon product from fly ash, whereby the hydrocarbon solvent is recycled into the recovery system.
Additional objects, advantages, and novel features of the invention are set forth in the description which follows and will also become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.